Method for manufacture of a package for the transport and/or storage of nuclear material, using the phenomenon of welding shrinkage

ABSTRACT

A method for the manufacture of a package for the transport and/or storage of nuclear materials, where the package includes an inner ferrule, an outerferrule and means of thermal conduction, and where the method includes: the construction of an essentially circular casing having at least one lengthways slit defined by two facing lengthways edges; and the welding of the facing lengthways edges, in order to obtain the outer ferrule, where this step is accomplished such that the welding shrinkage causes a compression stress of the means of thermal conduction between the ferrules, in a radial direction, and such that each welding bead only fastens the two edges which it links one on to the other.

DISCLOSURE

1. Technical Field

The present invention relates generally to the field of the manufacture of packages for the transport and/or storage of nuclear material.

In particular, the invention concerns a method for manufacture of a package for the transport and/or storage of nuclear materials, including an inner ferrule, an outer ferrule and also means for thermal conduction in contact with both the inner and outer ferrules.

2. State of the Prior Art

Traditionally, for the transport and/or storage of nuclear materials, storage devices, also called storage “baskets” or “racks”, are used. These storage devices, habitually cylindrical in shape and essentially circular in section, are suitable for receiving nuclear materials. The storage device is intended to be housed in the cavity of a package in order to form jointly with the latter a container for the transport and/or storage of nuclear materials, in which they are completely confined.

The abovementioned cavity is generally defined by a lateral body extending in a lengthways direction of the package, where this lateral body habitually includes two metal concentric ferrules jointly forming an annular space inside which are housed means for thermal conduction, together with a radiological protection device, in particular to form a barrier against the neutrons emitted by the nuclear material housed in the cavity.

The means of thermal conduction enable the heat released by the nuclear materials to be conducted outside the container, in order to prevent any risk of overheating, which might cause damage to these materials, an impairment of the mechanical properties of the materials constituting the package, or again an abnormal rise in pressure in the cavity.

The installation of these means of thermal conduction in the course of the method of manufacture of the package is often lengthy and difficult, and as a consequence relatively expensive.

An example of an embodiment is disclosed in document EP-A1-0 741 628. Thermal conduction ribs are installed in the inter-ferrule space, with an inner radial end attached securely to a part of contact of the inner ferrule, and an outer radial end in a recess of the outer ferrule, formed by the two ferrule sectors positioned edge-to-edge. For each rib it is the assembly by welding of the two ferrule sectors which creates the maintenance by clamping of the outer radial end of the rib, in the circumferential direction. As a consequence, this procedure requires as many ferrule sectors as there are thermal conduction ribs, and also requires the production of as many welding beads as there are ferrule sectors. Since the number of thermal conduction ribs is generally very high, able to reach, for example, several tens, the method of manufacture of the package is therefore greatly hindered, in terms of assembly time, by the installation and welding of the many sectors forming the outer ferrule which holds in place, by clamping, the thermal conduction ribs.

Moreover, the package manufacturing time is hindered further by the machining on the lengthways edges of the ferrule sectors, intended to form the recesses gripping the outer radial ends of the thermal conduction ribs.

Lastly, the performance of a package obtained in this manner, in terms of heat evacuation, is limited. Indeed, instead of an essentially uniform temperature of the outer ferrule, in the circumferential direction, on the contrary the existence of hot points on this ferrule is observed, in the area of the outer radial ends of the thermal conduction ribs.

SUMMARY OF THE INVENTION

The purpose of the invention is therefore to provide at least partially a solution to the disadvantages mentioned above, compared with the embodiments of the prior art.

To accomplish this, the object of the invention is a method for manufacture of a package for the transport and/or storage of nuclear materials, where the said package includes an inner ferrule, an outer ferrule and also means for thermal conduction in contact with each of the inner and outer ferrules, where the said method includes the following steps:

the construction, around the means of thermal conduction fitted to the inner ferrule, of an essentially circular casing having at least one lengthways slit defined by two facing lengthways edges; and

the assembly by welding of the facing lengthways edges, in order to obtain, starting from the said casing, the said outer ferrule, where this step is undertaken in such a way that the welding shrinkage causes a compression stress of the means of thermal conduction between the inner and outer ferrules, in a radial direction of the package, and in such a way that each welding bead obtained between two facing lengthways edges only fastens these two lengthways edges on to one another.

Thus, the invention makes use of the phenomenon of welding shrinkage, which is put to use to obtain the contact, or increase the intensity of this contact of the means of thermal conduction with the two ferrules, in the radial direction. In other words, the operation of assembly by welding allows, as a consequence of the welding shrinkage, a reduction of the perimeter of the casing designed to form the outer ferrule to be obtained, leading to a radial compression of the means of thermal conduction in the inter-ferrule space. This procedure leads to an essentially uniform radial stress of the means of thermal conduction between the two ferrules, in the circumferential direction, giving satisfactory contact along the length of the latter.

The principal benefit of this method lies in the simplicity of implementation and the short manufacturing time since, unlike the solution presented in document EP-A1-0 741 628, the number of angular sector(s) forming the casing is not conditional in any way on the number of thermal conduction elements extending radially in the inter-ferrule space, and this latter number can, indeed, be substantially higher. Here, it is stipulated that the angular sector(s) must be included as the constituent elements of the casing, defining the lengthways slits.

In addition, the method according to the invention is applicable whatever the design of the means of thermal conduction. Finally, it does not advantageously require any particular machining of the lengthways edges forming the slit(s) of the essentially circular casing.

With this regard, it is noted that the step of production of this casing may be implemented such that each slit extends discontinuously or continuously along the full length of this casing in the lengthways direction. By way of example, discontinuities may be created by points of tangency, such as welding points, between the facing lengthways edges, the aim being to pre-assemble the latter before the step of assembly by welding. This pre-assembly is sought in particular when many angular casing sectors are to define these lengthways edges, and to be arranged around the means of thermal conduction, in order to hold and position them relative to one another. Indeed, such sectors may be positioned one-by-one around the means of thermal conduction, each time pre-assembling the final angular sector on the previous sector, using the points of tangency mentioned above. This pre-assembly then proves to be particularly judicious when this step of manufacture of the casing is implemented with the package positioned essentially horizontally.

It is noted that in this particular case, in which a large number of angular casing sectors are arranged around the means of thermal conduction to form the said casing, the sectors can be held and positioned, before the step of assembly by welding, by the points of tangency, and/or by tools designed for this purpose, such as jacks positioning the sectors in order to reveal the said slits between the directly consecutive sectors. In this latter case the points of tangency are no longer required, and the lengthways slits are then preferentially continuous in the lengthways direction.

Moreover, it is noted that the casing formed from several sectors could be produced otherwise than by gradually arranging angular casing sectors around the means of conduction, such as, for example by positioning a prefabricated casing around these means of thermal conduction, using several sectors revealing the lengthways slits.

As will be recalled below, it is also possible to design a casing formed from a single angular section, therefore close to 360°, but which nonetheless reveals a lengthways slit.

As mentioned above, the step of welding is implemented such that each welding bead obtained between two facing lengthways edges only fastens these two lengthways edges on to one another. In other words, the outer casing obtained after welding is free relative to the elements located in the inter-ferrule space, against which it is pinned radially towards the interior. Each bead therefore provides no other fastening than that of the two edges which it links, whether it is positioned opposite the means of thermal conduction, the means of radiological protection, or again opposite any other means which may be installed in the inter-ferrule space.

During the welding shrinkage observed during the assembly step, a relative movement is created in the circumferential direction between the inner surface of the casing and the thermal conduction elements. The advantageous consequence of the fact that each welding bead linking two edges is not fastened on to the means of thermal conduction is that it does not excessively constrain the latter in the circumferential direction, thus preventing damaging the means of radiological protection which may be installed between these means of thermal conduction. The said means of thermal conduction are preferably chosen such that they have first parts of contact intended to be in contact with the inner ferrule, and second parts of contact intended to be in contact with the outer ferrule, where the said first and second parts are arranged in alternating fashion in the circumferential direction, with each second part of contact linked to the first two parts of contact being directly consecutive to it, using respectively two joining parts.

Thus, it is the two joining parts, preferably extending essentially radially from the inner ferrule to the outer ferrule, which provide the heat transfer to the second part of contact, which is supported on the inner surface of the outer ferrule. This second part of contact can extend over a given length in the circumferential direction, which limits the appearance of hot points on the outer ferrule, therefore favoring its temperature uniformity in this same direction.

Each unit formed by a second part of contact and its two associated joining parts preferably forms a pattern, where the successive patterns in the circumferential direction are linked to one another by the said first parts of contact.

In this configuration it can similarly be observed that each unit formed by a first part of contact and its two associated joining parts, positioned either side of the part of contact, forms a pattern, where the successive patterns in the circumferential direction are linked to one another by the said second parts of contact.

Thus, if a first unit formed by a second part of contact and its two associated joining parts, and a second, directly consecutive unit formed by a first part of contact and its two associated joining parts, are considered, then these two units have one of the joining parts in common. If the first unit forms a first pattern and the second unit forms a second pattern, then, preferably, the first and second patterns sharing one of the joining parts are preferably of the same shape, but arranged in an inverted fashion relative to one another, in the radial direction.

As an example, each pattern takes in its transverse section the shape of a slot or a wave. In the case of the slot this may be an essentially square, rectangular or parallelogram shape, without one of the sides, or again a trapezoid shape without one of its bases, either the larger or the smaller base.

Each first part of contact preferably has a length essentially identical to that of the second part of contact, even if different lengths could be chosen, without going beyond the scope of the invention.

As mentioned above, the said casing is made using one or more angular casing sectors, where the number Nf of lengthways slits, each intended to be the location of a welding bead, is identical to the number Ns of angular sectors.

The number Nf of lengthways slits is preferably lower than the number Np of second parts of contact belonging to the means of thermal conduction. The ratio between these two numbers is preferably between 0.02 and 0.5.

With this regard, the number Nf of lengthways slits, and therefore also the said number Ns of angular sectors, is less than or equal to four.

It is noted that each slit may equally be facing a first or second part of contact of the means of thermal conduction. If the positioning faces a second part of contact, it is however done preferably such that the welding bead made in the assembly step does not become secured to the outer ferrule on this second part of contact.

With this regard, as mentioned above, each second part of contact is, indeed, preferably intended to be simply supported on an inner surface of the outer ferrule, after implementation of the said assembly step, whether or not the second part is supported on a part of the casing having a welding bead.

Preferably, therefore, no additional element is used to fasten these elements pinned one on to the other.

As was explained above, during the welding shrinkage observed during the assembly step, a relative movement is created in the circumferential direction between the inner surface of the casing and some or all of the second parts of contact supported on this surface, and this movement therefore results from the reduction of the diameter in the casing designed to form the outer ferrule, after welding shrinkage. This movement ensures that the means of thermal conduction are not excessively constrained in the circumferential direction, notably with a view to preventing damaging the means of radiological protection, which are preferably arranged in the patterns of the means of thermal conduction.

To facilitate this relative movement each angular section has an inner surface shaped like the arc of a circle in its transverse section. Such an arc of a circle shape can moreover be adopted for the second parts of contact, even in the cases of patterns with a quadrilateral shape mentioned above. An essentially straight shape is also suitable.

Finally, given that each angular sector also preferably has an outer surface shaped like the arc of a circle in its transverse section, it is consequently easy to manufacture such sectors, of constant thickness, for example through a simple and inexpensive technique seeking to shape, using an appropriate tool, an initially flat sheet of metal, in order to obtain the desired arc-shaped sector.

Finally, it is noted that each casing sector may be made from a single part, or alternatively using several elements added attached to one another, for example by welding, preferably before the sector is positioned on the means of thermal conduction.

Other advantages and characteristics of the invention will appear in the non-restrictive detailed disclosure below.

BRIEF DESCRIPTION OF THE DRAWINGS

This description will be made with reference to the attached illustrations, among which

FIG. 1 represents a transverse section view of a container for the transport and/or storage of assemblies of nuclear fuel, including a package obtained by a method of manufacture according to the preferred embodiment of the present invention;

FIG. 2 represents a perspective view of the package in the course of the manufacturing method, after the step of production of the casing intended subsequently to form the outer ferrule of the package;

FIG. 3 represents a transverse section view of the package shown in FIG. 2;

FIG. 4 represents a partial transverse section view of the package, representing schematically the next step of assembly by welding of the lengthways edges of the casing; and

FIG. 5 represents a casing intended to form the outer ferrule of the package, according to an alternative embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Firstly, with reference to FIG. 1, a container 1 for the transport and/or storage of assemblies of nuclear fuel can be seen. It is with this regard recalled that the invention is not limited in any way to the transport/storage of this type of nuclear material. As an example, the invention can also be applied to the transport/storage of PuO₂ powder.

Container 1 comprises globally a package 2 forming the subject of the present invention, inside which is a storage device 4, also called a storage basket. Device 4 is designed to be positioned in a receptacle cavity 6 of package 2, as is shown schematically in FIG. 1, where it is also possible to note the lengthways axis 8 of this package, merged with the lengthways axes of the storage device and the receptacle cavity.

Throughout the disclosure the term “lengthways” must be understood as being parallel to the lengthways axis 8 and to the lengthways direction of the package, and the term “transverse” must be understood as being orthogonal to this same lengthways axis 8.

Container 1 and device 4 forming reception receptacles of the fuel assemblies are shown here in a horizontal/lying position, which is habitually adopted during the transport of the assemblies, and which differs from the vertical position for loading/unloading of the fuel assemblies.

Generally, package 2 essentially has a base (not represented) on which the device 4 is intended to rest in its vertical position, a cover (not represented) positioned at the other lengthways end of the package, and a lateral body 10 extending around and in lengthways axis 8, i.e. in the lengthways direction of container 1.

It is this lateral body 10 which defines the receptacle cavity 6, using a lateral inner surface 12, of essentially cylindrical shape and circular section, and having an axis merged with axis 8.

The base of the package, which defines the base of cavity 6 which is open in the area of the cover, may be manufactured from a single part with a part of lateral body 10, without going beyond the scope of the invention.

Again with reference to FIG. 1, it is possible to see in detailed fashion the design of lateral body 10, which firstly presents two concentric metal ferrules forming jointly an annular space 14 centred on the lengthways axis 8 of the package. This is, indeed, an inner ferrule 20 centred on axis 8, which is here of a double-thickness design, and an outer ferrule 22, which is also centred on axis 8. This outer ferrule 22 has the feature that it is formed by a number of angular sectors of ferrule 24, for example four such sectors, of equal angular extent, as has been represented in FIG. 1. These sectors 24 are assembled on to one another by their facing lengthways edges, by means of lengthways welding beads 26 linking these lengthways edges two-by-two.

The annular space 14 is filled by means of thermal conduction 16, and also by a radiological protection device 18 designed to form a barrier against the neutrons emitted by the fuel assemblies housed in the storage device 4. Thus, these elements are housed between inner ferrule 20, the inner surface of which matches the lateral inner surface 12 of cavity 6, and outer ferrule 22.

More specifically, the means of thermal conduction 16 have, alternating in the circumferential or tangential direction, first parts of contact 28 in contact with the outer surface of the inner ferrule 20, and also second parts of contact 30 in contact with the inner surface of the outer ferrule 22. In addition, each second part of contact 30 is linked at its ends with the first two parts of contact 28 which are directly consecutive to it, by means respectively of two joining parts 32 extending essentially radially from the inner ferrule to the outer ferrule, providing by this means the heat transfer to the second part of contact. The second parts of contact 30 extend over an angular length which may be essentially identical to that of the first parts 28, even if a different circumferential length could be used for the first and second parts 28, 30. As an indication, the circumferential length of each second part 30 can be such that its ratio with the perimeter of the inner surface of the outer ferrule 22 is greater than 0.01. This relatively great extent limits the appearance of hot points on the outer ferrule 22, and favours its temperature uniformity in the circumferential direction.

As can be seen in FIG. 1, each unit formed by a second part of contact 30 and its two associated joining parts 32 preferably forms a pattern 34, where the successive patterns are linked to one another by the first parts of contact 28. Similarly, each unit formed by a first part of contact 28 and its two associated joining parts 32 positioned either side of the latter forms a pattern 36, where the successive patterns 36 are linked to one another by the second parts of contact 30. The patterns 34, 36 which succeed one another, and which share the joining parts, preferably have the same shape, but arranged in a reverse fashion relative to one another, in the radial direction.

By way of example, each pattern 34, 36 takes in its transverse section the shape of a slot or a wave. In the represented case of a slot this has, for example, an essentially trapezoid shape without one of its bases, or again any other shape deemed appropriate. Parts 28, 30 can take in their transverse section the shape of segments of a straight line, or again arcs of a circle, preferably with a radius essentially identical to that of the surface of the ferrule with which they are in contact, in order to increase the extent of this contact and by this means improve the thermal transfer. Moreover, to ensure that such a contact exists, the means of thermal conduction 16 are held, or compressed radially between the two ferrules 20, 22, in a manner which will be described in detail in due course.

The radiological protection device 18, for its part, preferably takes the shape of multiple radiological protection units 40, housed in the recesses defined by the patterns 34, 36, in order to fill these recesses. Thus, each unit or assembly of units is demarcated circumferentially by two directly consecutive joining parts 32, and radially firstly by means of a first or second part of contact, and secondly by means of the ferrule surface facing this latter part of contact. The units 40 are made from any material known to the skilled man in the art to fulfil a radiological protection function with regard to neutrons, such as, for example, a material with a base of vinyl ester resin, and the means of thermal conduction 16 are, for example, made from a light alloy of the aluminium alloy type, preferably by drawing of sheet metal.

With reference at present to FIGS. 2 to 4, a method of manufacture of package 2 which has just been described is represented schematically.

The manufacture includes the production, around the means of thermal conduction 16 fitted to the inner ferrule 20, of a casing 22′ which is essentially circular, centred on axis 8, as is visible in FIGS. 2 and 3. This casing 22′ is made using several angular casing sectors 24′, the number Ns of which is four in this case. Indeed, each sector 24′ is intended to form, when the method is implemented, one of the outer ferrule angular sectors 24 described above. Thus, in this case also, sectors 24′ preferably have an inner surface and an outer surface which have respectively the shape of two arcs of a circle with the same centre, and therefore of essentially constant thickness. Furthermore, the angular extent is also essentially the same for each of the four sectors 24′. The latter are made either from a single part, or using several elements attached to one another, for example attached to one another by welding in the lengthways direction, preferably before the sector is positioned on the means of thermal conduction.

Sectors 24′ are arranged edge-to-edge around axis 8. Thus, between two facing lengthways edges 42, belonging respectively to two directly consecutive sectors 24′, there is a slit 44 intended to be the location of a welding bead, which will be made later during the manufacturing method. Thus, the number Nf of lengthways slits 44 is identical to the number Ns of angular sectors 24′, i.e. four in the present case. In addition, the number Nf of slits 44 is well below the number Np of second parts of contact 30, and the ratio between these two numbers may be between 0.02 and 0.5.

In the schematic views of FIGS. 2 and 3 the sectors 24′ have been represented without any means to hold them one to another, and at a distance radially towards the outside of the means of thermal conduction 16. Firstly, it is noted that a pre-assembly of the angular sectors 24′ is nonetheless preferentially undertaken in the area of the lengthways edges 42, in order to ensure that they are held and positioned one relative to another. Indeed, such sectors may be positioned one-by-one around the means of thermal conduction, each time pre-assembling the final angular sector on the previous sector, using points of tangency made in the area of the slit. Thus, at the end of the step of production of the casing 22′, each slit 44 can extend discontinuously along this casing, in the lengthways direction, and these discontinuities created by the points of tangency can take the form of welding points between the facing lengthways edges 42.

In addition, at the end of the step of production of casing 22′ the latter is preferably in contact with the means of thermal conduction 16.

The following step of the method consists in producing the assembly by welding of the facing lengthways edges 42, in order to obtain, from the casing 22′, the outer ferrule 22.

With reference to FIG. 4, this step is undertaken with conventional welding means 56 such that the welding shrinkage, observed during the production of the welding beads 26 in the slits designed for this purpose, causes a reduction of the diameter of the casing 22′, which then becomes the outer ferrule of package 2. The aim of this reduction of diameter is to obtain the contact, if it has not yet been made, or to emphasise the intensity of this contact, of the means of thermal conduction 16 with the ferrule 20 and the casing, in the radial direction, since these means 16 are subjected to a compression stress between these two elements. This procedure, which is represented schematically in FIG. 4, indeed leads to an essentially uniform radial stress of the means of thermal conduction 16 between the ferrule 20 and the casing 22′, the diameter of which is reduced, as is shown schematically by the arrows 50. The observed radial compression may lead to an elastic deformation of the second parts of contact 30 with a view to increasing the surface of this contact with the casing 22′, and thus providing better thermal transfer.

In this preferred embodiment, each welding bead 26, which extends essentially over the entire length of casing 22′, is intended to be facing, and in contact with, one of the second parts of contact 30. Nonetheless, it is however made such that the welding bead 26 does not become secured to the casing 22′ on this second part of contact 30. Indeed, each second part of contact is simply supported on the inner surface of the outer ferrule 22 after the implementation of the assembly step. Thus, during the welding shrinkage observed during the assembly step, a relative movement is created in the circumferential direction between the inner surface of the casing 22′ and some or all of the second parts of contact 30 supported on this surface, and this movement, represented schematically by arrow 52 in FIG. 4, therefore results from the reduction of the diameter of the casing 22′ designed to form the outer ferrule 22, after welding shrinkage. Conversely, it is stipulated that the first parts of contact 28 of the means of conduction 16 are preferably assembled securely on the inner ferrule 20, for example using welded studs and nuts, or any other similar means.

According to an alternative embodiment represented diagrammatically in FIG. 5, casing 22′ is formed from a single angular sector 24′, therefore close to 360°, revealing a lengthways slit 44 between its two facing edges 42, 42. The step of assembly by welding is then undertaken in a manner similar to that set out above, making a welding bead in the slit designed for this purpose, in order to obtain the desired welding shrinkage.

Naturally, various modifications can be made by the skilled man in the art to the invention which has just been described, solely as non-restrictive examples. 

1. A method for manufacture of a package for the transport and/or storage of nuclear materials, where the said package includes an inner ferrule, an outer ferrule and also means for thermal conduction in contact with each of the inner and outer ferrules, where the said method includes the following steps: the construction, around the means of thermal conduction fitted to the inner ferrule, of an essentially circular casing having at least one lengthways slit defined by two facing lengthways edges; and the assembly by welding of the facing lengthways edges, in order to obtain, starting from the said casing, the said outer ferrule, where this step is implemented such that the welding shrinkage causes a compression stress of the means of thermal condition between the inner and outer ferrules in a radial direction of the package, and in such a way that each welding bead obtained between two facing lengthways edges only fastens these two lengthways edges onto one another.
 2. A method according to claim 1, characterised in that the said means of thermal conduction are preferably chosen such that they have first parts of contact intended to be in contact with the inner ferrule, and second parts of contact intended to be in contact with the outer ferrule, where the said first and second parts are arranged in alternating fashion in the circumferential direction, with each second part of contact linked to the first two parts of contact being directly consecutive to it, using respectively two joining parts.
 3. A method according to claim 2, characterised in that each unit formed by a second part of contact and its two associated joining parts forms a pattern, where the successive patterns in the circumferential direction are linked to one another by the said first parts of contact.
 4. A method according to claim 2, characterised in that each pattern takes in its transverse section the shape of a slot or a wave.
 5. A method according to claim 1, wherein each part of contact has a length essentially identical to that of each second part of contact.
 6. A method according to claim 1, wherein said casing is made using one or more angular casing sectors, where the number Nf of lengthways slits, each intended to be the location of a welding bead, is identical to the number Ns of angular sectors.
 7. A method according to claim 6, wherein the number Nf of lengthways slits is strictly lower than the number Np of second parts of contact belonging to the means of thermal conduction.
 8. A method according to claim 6, characterised in that the said number Ns of angular sectors is less than or equal to four.
 9. A method according to claim 6, characterised in that each angular sector has an inner surface with, in its transverse section, the shape of an arc of a circle.
 10. A method according to claim 2, wherein each second part of contact is intended simply to be supported on an inner surface of the outer ferrule, after implementation of the said assembly step. 